NERVE BLOCKS

Definition – A nerve block is the insertion of a needle close to a nerve – sometimes requiring fluoroscopic X-ray control – in order to inject a solution to cause a neural blockade (temporary or permanent).

«  About 7% of hospice in-patients with cancer pains benefit from some kind of nerve blocking procedure.

Indications for a nerve block:

• Localized pain
• Pain uncontrolled by drugs plus psychotherapy
• Pain unresponsive to chemotherapy or radiotherapy
• No coagulopathy (stop warfarin!)
• No tumor at site of injection
• Willing patient and family
• Possible side effects explained
• Benefits explained (so expectations are realistic)

1. Celiac plexus block (see Celiac Plexus Block)

2. The paravertebral nerve block is used to control pain from the thoracic spine and chest wall. Although sometimes called a somatic block it also involves the sympathetic supply. It is often more useful than an intercostal block because it covers several segments.

It can be particularly useful for:

• Chest wall pain
• Mesothelioma pain
• Vertebral metastases
• Esophageal pain (T3, 4, 5)
• Rib fractures
• Post-thoracotomy pain
• Post-herpetic neuralgia

X-ray control is used to place the needle (under local anesthetic) in the paravertebral space. Complications are rare.

3. Epidural steroids can be given into the epidural space which lies inside the vertebral bone, but outside the dural sac and the cerebrospinal fluid (CSF). A single injection of steroids (methylprednisolone 80mg to 120mg) into the epidural space can be useful for root irritation after vertebral collapse. It can be given at any level. It can give pain relief within 48 hours that can last for weeks.

4. Epidural morphine infusion can be useful if a patient on high dose morphine has troublesome drowsiness. It is a safe procedure that should normally be considered before other more destructive nerve blocking procedures. The incidence of morphine side-effects is reduced compared to other routes. The starting dose is normally around 25% of the regular oral dose (for example, a patient on 60mg oral morphine every 4 hours would be given 15mg epidurally as a bolus, which may last 8 to 12 hours). In one study of epidural morphine in 37 patients with intractable cancer pain the average daily dose was 24mg of epidural morphine; the duration of epidural use ranged from 2 to 177 days. Visceral and bone pain responded better than nerve pain. If the analgesic effect is lost despite high epidural doses of morphine (over 100mg per day), it is sometimes possible to regain the effect by stopping morphine for 48 hours (to “rest” the receptor), using epidural clonidine in the interim (as a substitute analgesic), then stopping clonidine and re-starting morphine in lower doses than before. Alternatively, continue morphine and consider adding dilute (0.125%) bupivacaine (usually 10mls every 2 to 3 hours) which can reduce pain without causing leg weakness or urinary retention. (see Spinal Opioids)

5. Epidural local anesthetic (widely used in obstetrical practice) has the disadvantage of causing weak, numb legs and urinary retention. It can have a place for severe pelvic pain or painful pressure sores when the patient is already immobile and has a urinary catheter.

6. Psoas compartment block is useful for pain due to infiltration of the lumbosacral plexus, or for hip pain (either arthritic or malignant). The psoas compartment lies anterior to the transverse processes of the lumbar vertebrae. It can be identified by injecting dye under X-ray control. Local anesthetic and steroids are injected. Reversible leg weakness sometimes occurs. Pain relief is usually good.

7. Intrathecal neurolysis is the injection of a neurolytic agent into the CSF to destroy sensory nerve roots (but motor nerve roots are also at risk). The spread of the neurolytic agent is controlled by gravity. The patient is positioned so that the anterior (motor) roots are rotated away from the neurolytic agent. It is usually reserved for patients with perineal pain (for example, a pelvic recurrence following surgical abdomino-perineal resection) who are already immobile and have a permanent urinary catheter, since there is a risk of causing leg weakness or urinary incontinence. Leg weakness may be temporary (for 2 to 3 days) but can be permanent. Some centers use intrathecal neurolysis to treat thoracic or upper lumbar nerve root pain.

8. Caudal (sacral) extradural nerve blocks can reduce sciatic or perineal pain. The needle is inserted via the sacral hiatus to gain access to the trans-sacral canal. 0.5% bupivacaine is used initially, and if the effect of the block is acceptable a neurolytic agent can then be used.

In one series of caudal blocks (using a cryoprobe) for sciatic pain, 12 out of 17 patients had pain relief. Although its main use is in the management of acute pain, it has been used with success for patients with cancer pain. It can be repeated.

Cryoanalgesia is the application of a cryoprobe to a peripheral nerve. It blocks nerve conduction and relieves pain. It produces a reversible block for a median duration of 11 days (range up to 224 days).

The cryoprobe is applied under local anesthesia. It contains a nerve stimulator for accurate positioning. The probe releases liquid nitrous oxide which freezes to -60°C. The axon dies but regrows at 1mm/day, giving a more prolonged block than local anesthetic but without the risk of painful neuralgia following neurolytics. Occasionally relief lasts several months, despite early recovery of motor and sensory function.

9. Local infiltration (see Fractures)

10. Brachial plexus block is rarely indicated. A continuous infusion of local anesthetic can occasionally be useful for cancer pain, but it causes a numb, paralyzed arm which the patient may find more unpleasant than the pain. Neurolytic agents are contraindicated, partly because the patient may want the effect reversed, and partly because of high incidence of painful neuritis at this site.

Discussion:

«  The indication for a nerve block is pain irrespective of prognosis. A nerve block may be worthwhile even in a very ill patient if pain is not otherwise controlled.

A graduated approach to pain control is adopted, so that non-invasive low-risk techniques are used before nerve blocks are considered. A nerve block should never be used as sole therapy. In certain situations, however, if a nerve block is known to be very effective it should be considered early (for example, a celiac plexus block for pancreatic pain not responding to titrated doses of morphine).

«  Nerve blocks are an adjunct to other treatments. They can reduce pain but rarely abolish pain.

«  Anti-coagulant therapy is an absolute contraindication to nerve blocks.

A reversible diagnostic block should be performed with a local anesthetic before neurolytic agents (phenol or absolute alcohol) are used to give long-lasting blockade. (This is best explained to the patient as “a trial run”, rather than an “experiment”.) This is to establish distribution of pain and observe side effects.

A diagnostic somatic block causes decreased sensation to pinprick. If the patient continues to complain of pain despite evidence of a block, or if the patient claims pain relief with no evidence of a block, it is futile to proceed to a therapeutic block. Occasionally a local anesthetic block can last weeks or months, for unknown reasons.

Bupivacaine (0.5%) is the best local anesthetic for nerve blocks. The duration of action is 8 to 12 hours, and there are few side effects up to 2mg/kg.

The dose of morphine should be reduced or stopped following a nerve block, and the patient observed for 24 hours. Morphine requirements may decrease as pain lessens. Drowsiness or even respiratory depression can occur if the patient remains on more morphine than is now required for the pain. One method is to stop morphine and promptly re-titrate (use an initial low dose of morphine if there is still some pain and increase the dose every 4 hours as necessary). (see Morphine)

It is essential that the actual nerve block procedure be painless. The object is to relieve suffering. The procedure should normally be carried out under local anesthetic and sedation with IV midazolam, which also causes retrograde amnesia.

Detailed explanation is essential. The terminally ill patient is vulnerable, and needs to have a sense of control over medical intervention. Often the patient has already adjusted to the idea of having no more invasive procedures, and it is therefore psychologically difficult to undergo a nerve block. However “routine” nerve blocks may be to the doctor, if the patient is dubious it is usually better to postpone the procedure. The family should also be clearly told what is being considered, and why.

The author and publisher have taken precautions to ensure that the information in this book is error-free. However, readers must be guided by their own personal and professional standards of good practice in evaluating and applying recommendations made herein. The contents of this book represent the views and experience of the author, and not necessarily those of the publisher.